Won't run below 1500 RPM

Okay, this is going to be a lengthy post, and if this is a common issue I apologize in advance. I posted this on another forum, but the HAMB has a lot more members so I figured I'd give it a shot. In short, my car refuses to idle, or even run below 1500 rpm. (For reference, its a '56 Chevy with a 283, with a mid 80's Quadrajet sitting on top)

To get the whole story, I had the car running and driving for awhile. I was using a Quadrajet of unknown origin, but looked rebuilt, a Mallory Unilite with a stock coil, and stock plugs and wires. The car ran pretty good, but it was tough to get started even if warm.

On Valentine's Day, the car died. I mean, it simply just off at a stoplight as if I had turned the key off. Wouldn't restart, would crank and crank and crank but no fire whatsoever. I could see gas squirting down the carb, but I don't think I had any ignition. I didn't verify it however, as I was tired of the hard starts and wanted to go with an HEI setup anyway.

So I bought a small diameter HEI distributor, a Flamethrower coil, Accel wires and some brand new AC Delco plugs. I put the distributor back in the same position that the Unilite was in, rewired the ignition switch and coil to eliminate the ballast resistor, and put in the new wires and plugs. (Gapped to 0.045")


When I went to start the car, it would fire, but refused to run. I played with the timing a bit, but it didn't change - it would fire, rev up for a split second, and die. As I played with the timing, I finally got it to the point where it was backfiring through the carb so I knew I had gone to far. (Also got some <NOBR>smoke</NOBR> out of the carb, which was a little scary). This happened quite a few times while I was playing with the timing.

At this point, I decided to start over. Disconnected the coil, pulled the valve cover and #1 spark plug, and brought the #1 cylinder back to TDC with both valves closed. Set it for 4 deg BTDC, and lined up the #1 spark plug wire with where the rotor was and put it all back together.

Same story. Would fire, but not run. Played with the timing a little, despite being sure I had set it to where it needed to be. Finally got it to run, but only by holding the gas down to the point where the engine would rev to 3 grand when fired. (I don't like doing that).

So, it runs, but only if I keep the engine revved up. It seems to run pretty well, if I hold it at 2k and give it some gas, it revs right up to 3k no problem. However, if I let off the throttle, it dies very quickly. If I slowly ease off the gas, it dies once it gets to about 1500 rpm. Doesn't really sputter, or idle badly for a few seconds, it just simply dies.

Thinking it might just be cold, I held it between 2-3k for a bit and got the temp up. Didn't change anything, simply won't idle. I checked all my vacuum connections, everything is connected or plugged up that should be.

So I am hoping for some advice from someone more experienced than me. I am thinking maybe my carburetor is my problem now. It did backfire though the carb a lot when I was trying to get it to run (plus the smoke), is it possible I damaged something in the carb? (I very carb inexperienced).

I wish I had a known good carb to test it, but unfortunately I don't.

Anyway, any input is appreciated. I was hoping to enjoy the car this weekend (its really nice out) but I guess not.
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get yourself a vacuum gauge and see what it says as it will reveal lots! even though you felt your post was long thanks for not cramming it together. makes you wonder sometimes if folks can't write how the fuck can they fix anything!

 

When I put an HEI on my 235, it'd hardley idle below 1200 rpms, had to change the wiring to and from the ballast resisor...may have even had to bypass the ballast resistor (it was a looong time ago). IIRC original wire was 16 gauge and I had to run 14ga....seems the generator didn't have enough output at idle to keep the HEI alive.

 

Sounds as though there's a vacuum leak.
55 Dude is right, get a vacuum gauge, and start checkin'

 

I vote vacuum leak, and a big one...I'd also check the voltage input going to the HEI as suggested, it might be too weak...oh, and rebuild that damned Quadrajet, the low-speed circuit might be plugged, so you have to keep the RPM's up high enough to stay on the high-speed circuit...good luck...

 

Might be the plugs leaking in the bottom of the float bowl. When my 66 Chevy had a Qjet on it it was very hard to start when cold, float bowl was empty by morning, but alot easier when warm. Found out somewhere about the leaking plug problem on Qjets, put on a Holley and end of problem. A common fix for the leaking plugs is JB Weld. Could be someone fixed yours and the stuff came off.

 

is it whistling or hissing? try spraying some carb cleaner around the base of the carb and vacuum lines.

 

Without reading the entire post my initial thought is vacuum leak.

Then I read and realize that you also screwed up by running stock wires with the Uni-Light. By stock I assume that you mean wire core plug wires?
They are real sensetive to plug wires.

That said you should have alleviated the spark problem with the new dizzy. Unless of course it is an accell electronic. In which case you cannot rule out electrical problem.

Anyway at first blush and without running any tests myself I'm going to stay with vacuum problem as a starting place.

 

Had a similar problem on a 327 with a q-jet(rebuilt), it was the accelerater pump jamming (open I think, it was about 15 years ago). pulled the top off the carb and cleaned the shaft with scotchbrite, problem solved.

 

+1 for vacuum issue. If your Q jet is 80's it probably has a few external diaphragms on it, make sure they are not cracked or leaking, especially if they are plastic.

Also you do want to check out your carb after the backfiring, but Q jets are relatively resistant to backfiring (older holleys are not) but it is possible your part-throttle metering system could be stuck open and flooding. This can often be re-set by smacking the bump just in front of the airhorn with a screwdriver handle.

Is it possible the distributor is off by 1 tooth?
A buddy of mine this and he could get it to run well but only in a certain rpm range though... If you have a timing light try having someone turn it over and if its way adv./ret. it'll be obvious.

 

I believe any and all of the electronic distributors need a full 12 Volts to the coil w/o a ballast resistor in the line.

I'm another one who thought vacuum leak right off the bat. You didn't say but what manifold are you running under the QJ? And if it isn't a manifold that is designed for a QJ do you have an adapter plate on it that may have a gasket leaking?
I'd also go along with leaking vacuum brakes on the carb if it has them as that is a common problem with these.

And don't overlook vacuum leaks inside the car or in the line to the modulator if it is an automatic and has one.

 

My Mallory Unilight needs a ballast resister unless it's a GM car with a resistance ignition wire.

 

The brake booster on my van leaks something terrible, and it takes forever to get that thing running without it dying at just above high idle...I'm thinking you have a vacuum leak coupled with bad timing...fix the leak and then set the timing and you'll be good...

 

Thanks for the replies guys. My father offered to let me borrow the carb off his Olds, so tomorrow I will try that. For reference, its a 283, the intake is Q-jet specific. Its a manual trans car with manual brakes and manual everything else. The only vacuum operated things are the vacuum advance and the vacuum hose that runs between the PCV valve on the back of the carb to the PCV adapter thingy on the back of the block. This setup worked fine with the other distributor.

I'll also check again more closely for any vacuum leaks. I was able to borrow a vacuum gauge, but I haven't gotten a chance to try it yet. What should I look for on the gauge, I've never taken any vacuum readings on an engine before. I am assuming if there was a really bad vacuum leak, it would barely read anything, but under normal conditions, how much vacuum would a stock SBC pull?

I appreciate the replies so far. I will let you guys know what I find.

 

i would run 12v direct from battery to HEI first before you change the carb. i had the leaky Q-jet but all that resulted was low float symtoms and had to prime to start but ran fine. spray some wd-40 around carb base.

 

55 dude is right. HEI needs 12 volts with no Ballast Resistor or High Resistance Wire. Also the Vacuum Gauge will tell you what's going on. I agree with everyone here about the Vacuum Leaks to Spray Carb Cleaner at the Carb Base, and all the Vacuum Lines to see if your Idle Changes.
Good Luck.

 

On a fresh ford 302 roller motor I got 23 inches of vacuum at 700 rpm. I was able to dial down the rpm's to 300 and the motor was still running. All that on an beat up Holley 600 I had laying around which I also rebuilt.

 

I definitely have 12V to the coil; I ditched the ballast resistor. I also verified that I had 12V to the coil with a voltmeter, both while cranking and when my dad started it and kept it running long enough for me to check.

I will also try the carb cleaner spray thing too. Thanks for all the good suggestions, I'll post what happens.

 

With the car not running open the top butterflies on the secondaries, with a flashlight make sure the bottom butterflies are tightly shut they might be slightly hung open

 

it an idle you should pull 15-18 inches of vacuum, it will decrease as the rpms go up. your looking for very low or a bouncing needle on the gauge. quads aren't prone to backfire problems like holleys, you could have an internal vacuum leak, could be over rich (sunk float, sticky needle...) but that you could smell and see black smoke from the exhaust.

 

I forgot that one and it is a good suggestion.

There is a vacuum gauge chart in almost every Motor manual and a lot of repair manuals.

Here's a good one http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

 

Okay guys, figured it out. The new distributor was no good, out of the box.

Since I noticed that the car's previous owner had clearanced the firewall (using a large hammer) for a normal sized HEI distributor, I dropped one in. (I have had one in my shed for years now, waiting for a use).

After some wiring and adjustments, the car fired right up and rain beautifully with that old distributor that has been lying around for years. Haven't even set the timing with a light yet and it already runs better than it ever has.